Radikal Therapeutics (RTX) is developing a novel pharmaceutical therapy for prematurity that targets a developmental deficiency in thioredoxin (Trx), a protein that serves as a central regulator of cellular reductant status. Trx is expressed t levels in the fetal lung too low to adequately counter the redox stress of extrauterine life, and i thus relatively deficient in extreme prematurity. The low level of Trx in alveoli is thought to pla a major role in the susceptibility of very low birthweight (VLBW) premature infants to neonatal respiratory distress syndrome (RDS). Our current understanding of RDS and its evolution into bronchopulmonary dysplasia (BPD) is consistent with a mechanism of injury produced by an excess of superoxide within the lung parenchyma. Compounding this increase in superoxide production is the developmental deficiency of anti-oxidant proteins, such as superoxide dismutase and Trx, that are present in insufficient quantity at an early gestational age. Our innovation is based upon the administration of a novel agent, R-908, a prodrug of a Trx mimetic (R-901) that substitutes for the deficient Trx protein and restores intracellular redox status. R-901 is a thiol-rich tripeptide closely analogous to the native conserved Trx motif and exhibits extraordinary potency: R- 901 is 450-fold more potent than N-acetyl cysteine in the protection of cultured cells from oxidant stress. In a murine ovalbumin model of pulmonary inflammation model, R-901 reduced histologic injury, diminished neutrophil infiltration, attenuated tissue oxidation, blocked pro-inflammatory cytokine expression and nuclear translocation of NF-?B, diminished the degradation of the anti-inflammatory cytoplasmic protein I?B, and restored the balance of reduced and oxidized forms of glutathione. In a murine LD60 model of redox stress, induced by acute Cl2 inhalation, post-insult administration of R-901 eliminated all mortality. To overcome shelf instability of the free thiol groups of R-901, we have invented a stable dithioester prodrug (R-908) that releases R-901 in vivo. R-908 will now undergo evaluation in a clinically-relevant rat pup model of BPD. Aim #1: Define the pharmacokinetics (PK) of R-908 in newborn rats. We will define the plasma and tissue PK profile of R-908, and its metabolite R-901, in 2-day old rat pups. Aim #2: Establish that R-908 attenuates changes of pulmonary vascular and alveolar structure in a hyperoxic model of BPD in neonatal rats; 2-day old rat pups will be subjected to hyperoxia for 10 days. R-908 will be administered over a broad dose range for 3 weeks, a period characterized in this model system by progressive lung fibrosis, pulmonary arterial hypertension (PAH), and hypoalveolarization. Lung tissue taken at necropsy will be analyzed for pulmonary vascular structure and growth, alveolarization, lipid peroxidation (malondialdehyde), reduced and oxidized glutathione (GSH, GSSG), peroxynitrite formation (3-nitrotyrosine), poly(ADP-ribose) formation, fibrosis (Mason Trichrome staining for collagen), R-908 and R-901 levels, and pro-inflammatory gene expression. The heart will undergo morphometric analysis for evidence of PAH (as shown by right ventricular (RV) hypertrophy).